A positioning system utilizing radio signals transmitted from artificial satellites such as GPS (Global Positioning System) satellites (hereinafter, referred to as “satellite positioning signals”) has a problem that the positioning accuracy deteriorates or positioning becomes impossible when a communication terminal such as a GPS receiver enters an area where the satellite positioning signals are not able to be received such as an indoor area, and an underground mall.
As a measure to solve this problem, for example, Patent document 1 discloses a technology of providing a positional information transmitter (indoor transmitter) transmitting positional information indicative of a position (hereinafter referred to as the “positional information signal”), using a signal compatible with one of signals having frequencies (for example, center frequency: 1.57542 GHz), a signal according to the modulation mode (specifically, BPSK (Binary Phase-Shift Keying), a signal according to the multiple access mode (specifically, CDMA (Code Division Multiple Access) in the Direct Sequence Spread Spectrum mode), etc., used in the GPS, to an area where the satellite positioning signals are not able to be received such as an indoor area or an underground mall and allowing a communication terminal such as a portable telephone to acquire the own current position from the received positional information signal. Further, Patent Document 2 and Non-Patent Document 1 disclose a technology of reducing the time required for acquiring the positional information.
In positioning with the positional information signal as described above, unlike positioning with the satellite positioning signal, the communication terminal determines that the position contained in a received positional information signal is the current own position without performing complicated positioning calculation. Accordingly, it is necessary that the positional information transmitters are arranged at intervals commensurate with the required positioning accuracy and the outputs are adjusted so that the positional information signals are not detected in areas remote from the place where they are transmitted. For example, when the positional information transmitters are arranged at intervals of 10 m, the outputs are adjusted so that the positional information signals are detected in a radius ranging from 10 m-15 m. Depending on the transmitter's sensitivity or the like, however, the signals are detected in a greater radius in some cases.
As described above, in positioning with the positional information signal, a plurality of positional information transmitters are disposed adjacently to each other. To avoid interference of the positional information signals with each other, it is desirable that the adjacent positional information transmitters do not use the same PRN (Pseudo Random Noise) number for selecting a code pattern in spread spectrum. The number of PRN numbers that are available indoors, however, is limited to about ten (10).    Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-278756    Patent Document 2: Japanese Patent Laid-Open Publication No. 2009-133731    Non-Patent document 1: User Interface Specification for Quasi-zenith Satellite System (IS-QZSS) V1.0 Annex Terrestrial Complementary Signal (IMES)